The present invention relates to an electric motor and, more particularly, to a miniature electric motor having a motion translator mechanism built in for converting the rotary motion of the rotor into a linear movement.
U.S. Pat. No. 3,984,709, which issued to the same inventor on Oct. 5, 1976, and is assigned to the same assignee of the present invention, discloses a miniature electric motor which comprises a substantially cylindrical casing of soft magnetic material having one end closed, a power output shaft rotatably extending through the closed end of the casing in coaxial relation to the casing, at least one rotor of soft magnetic material rigidly mounted on the power output shaft within the casing, a stator having the form of an annular permanent magnet and magnetized around its circumference to provide alternating north and south poles at equal angular spacings and rigidly mounted on a support base which additionally serves as a closure for closing the opening at the other end of the casing, a yoke member of soft magnetic material cooperating with the casing to form an alternating magnetic circuit with the rotor, and an annular exciting coil is housed within the casing in coaxial relation to the power output shaft and positioned adjacent to the rotor and the outside rotor boss and the yoke member.
The rotor has a flat circular portion lying in a radial plane perpendicular to the longitudingal axia of the power output shaft and facing the magnetized face of the permanent magnet stator, and also has a plurality of polarizable teeth in angularly equally spaced relation to each other. The yoke member, exciting coil and permanent magnet stator are positioned relative to the rotor so that a magnetic flux developed by the exciting coil, when the latter is energized, combines with the above described elements to form a magnetic loop with the polarizable teeth. Consequently the adjacent polarizable teeth on the rotor are magnetized in opposite polarities in response to the magnetic field produced by the energized exciting coil, the polarity of each of the polarizable teeth alternating in response to alternation of the magnetic field.
The electric motor described above and disclosed in the previously mentioned U.S. Patent is satisfactory in that the power output shaft can be precisely rotated through a desired angular displacement in response to a train of pulses applied to the exciting coil, each of the pulses representing a specifically defined displacement of the rotor. However, when the electric motor of the above described construction is applied to move an element in a linear direction, it is required that the driven element be coupled to the power output shaft of the electric motor through a motion translator which is generally composed of a rack and pinion.
The employment of the motion translator of rack-and-pinion construction not only increases the size and manufacturing cost of the drive unit as a whole including the electric motor, but also poses the problem of backlash which tends to occur between the pinion and the rack.
An alternative to an electric motor combined with an external motion translator for moving a driven element in a linear direction is a solenoid operated plunger. However, because of its operation, the solenoid operated plunger cannot be employed in applications in which the driven element is desired to be precisely stepped in a linear direction and, therefore, such a solenoid operated plunger cannot be employed as a substitute for an electric motor combined with an external motion translator.